Oil wells are generally produced using a rotary drill to make the hole. The drilling rig typically consists of a base upon or over which a derrick or tower rests. In the rotary drilling method, a drill bit is attached at the end of a string of drill pipes. This drill string is rotated causing the bit to bore into the earth to produce the hole. Oil drilling operations use drill pipes which are typically 30 feet long and about 5 inches in diameter. They are joined to one another by heavy pipe threads. The pipe just above the drill bit is much heavier than the other pipes and is called a drill collar. The drill collar is used to supply the proper amount of force onto the drill bit for drilling.
The drill string is pendulously supported by a movable hook mounted within the tower so that the total weight of the drill string does not bear upon the drill bit. The force on the bit is therefore independent of the length of the drill string but is generally determined by the weight of the collar. Without such support the weight of the entire drill string, which can easily exceed several hundred tons, would ruin the drill bit. Therefore it is possible for the drill bit to exert downward pressure on the earth being bored while essentially the entire drill string is being pendulously supported by the hook because of the great lengths and weights involved and the elasticity of the materials used. An analogous example would be to hold over a table one end of a rubberband which has a lead weight attached to the opposite end. By allowing the lead weight to just touch the table top, the lead weight partially bears against the surface; however the weight of the rubberband, as well as a portion of the weight of the lead weight, is supported by the person holding the rubberband and not by the surface.
The drill string is rotated by a rotary drill table mounted on the base. The rotary drill table engages a typically square or hexagonal cross-sectional pipe, known as a kelly, to turn the drill string. Most of the prior art drilling rigs support the hook within the tower by a block and tackle cable system actuated by a winch. As drilling progresses, the winch pays out cable allowing the hook to descend, thus allowing the kelly to slide through a complementarily shaped hole in the rotary drill table. When the top of the kelly is near the rotary drill table, the drill string is wedged into place, the kelly is disconnected from the top drill pipe, and a new length of drill pipe is added to the string. The kelly is then reconnected to the top of the new drill pipe and the hook is attached to the kelly after sufficient cable has been rewound on the winch thus resuspending the drill string. Drilling operations can then resume.
This sequence will be repeated over 600 times for a 20,000 foot well. Further, when the drill bit becomes dull, the entire drill string must be removed to replace it. Thus the cable must be payed out and taken up on the winch a great number of times when drilling a single hole.
One problem with the above described winch, block and tackle suspension arrangement is that the cable is always under tension and that it is wound on rewound on the winch hundreds of times while drilling a well. The brakes and clutch on the winch wear out relatively quickly under such conditions. Also the likelihood of cable failure is great because of these operational demands. Because the cable is wound on the winch, it cannot easily be inspected. It is unfortunately not surprising that a primary cause of accidental deaths in drilling operations is due to a cable breaking during the operations. The resulting ends of the cables, because of the great stress to which they were subjected, whip around in an unpredictable manner with great force and cause great destruction to anything in their path.
Another problem is a result of the block and tackle apparatus situated at the top of the tower. These multiple lines and pulleys occupy a great amount of space on the tower and interfere with the ease of mounting and dismounting the drill pipes to and from the drill string.
The following U.S. Patents may be of interest to the reader: U.S. Pat. Nos. 3,273,861 and 3,502,535.